One known control system for an direct direct injection internal combustion engine equipped with an exhaust gas recirculation device is disclosed in, e.g., JP,A 2-245460. In the known control system, only internal EGR gas is returned to a combustion chamber in a low load range, and external EGR gas is returned to the combustion chamber in a high load range.
JP,A 6-323201, for example, discloses another control system for an internal combustion engine in which an air-fuel ratio is controlled so as to provide a larger air surplus rate in a low load range than in a high load range, the engine including a first EGR device for returning high-temperature exhaust gas to a combustion chamber and a second EGR device for returning low-temperature exhaust gas to it. In such an internal combustion engine, the high-temperature exhaust gas and the low-temperature exhaust gas are both returned to the combustion chamber in the low load range, and a total recirculation rate of the exhaust gas to the combustion chamber is reduced in the low load range as compared with that in the high load range.
By returning exhaust gas to the combustion chamber as disclosed in those known control systems, the maximum temperature in a cylinder is lowered and therefore the amount of NOx can be reduced. In particular, when external EGR gas, i.e., low-temperature exhaust gas, is returned to the combustion chamber, the resulting effect is more noticeable and the amount of NOx is reduced as the amount of external EGR gas increases.
Also, when internal EGR gas, i.e., high-temperature exhaust gas, is returned to the combustion chamber, the temperature in a cylinder is increased and therefore the effect of reducing the amount of NOx is lessened in this point. On the other hand, because atomization of a fuel spray is promoted or evaporation of fuel attached to wall surfaces of the combustion chamber is promoted, the amount of HC can also be reduced along with a reduction in the amount of NOx. Thus, recirculation of the internal EGR gas is effective in reducing the amounts of both HC and NOx.